The present device relates to a miniature single lens reflex camera using a film cassette of small size.
There are several problems to be solved before achieving a practical single lens reflex camera of the type which not only uses a small film cassette having film feed and take-up chambers projecting forward from both sides and connected to each other by means of a bridge, but also includes a shutter of the slide type (or the so-called "guillotine shutter") arranged in front of said film cassette, and does not have a bulged portion often found in single lens reflex cameras for accommodating a roof-shaped prism.
One of the problems is that the photographing lens system, which is positioned in front of either a translucent mirror or a retractable totally reflecting mirror must have a large distance (so-called "back focus length") between its rear end and the frame position of the film in comparison with the size of the frame. The relatively long distance is necessary to provide room for the angled mirror, which reflects light to the finder optical system and must be positioned between the photographic lens system and the shutter, and for the projecting film feed and take up reels of the cassette. The latter provide a limit on the closeness of the film frame to the shutter. Under such conditions it is quite difficult to assemble the photographing lens system as above even with the use of a common lens of retrofocusing type. Although not impossible, this assembly requires excessively high techniques in the design of the lens system and becomes accordingly expensive.
Another problem is due to the attempts to assemble a finder optical system without formation of the above-mentioned bulged portion as is common in single lens reflex cameras. It is a common practice to provide a basic construction such that the light is guided sideways before and along the shutter, turned by a prism, and further guided backward into an eyepiece lens along the side of the film cassette. In such case, it is also common to focus the light by a first lens and then convert the image to an erect image by a second lens. This type of construction generally lengthens the optical path so much that the resultant finder optical system is not suitable for a small camera.